Sulfur recovery refers to the conversion of hydrogen sulfide (H2S) to elemental sulfur. Hydrogen sulfide is a byproduct of processing natural gas and refining high-sulfur crude oils. The conventional method of sulfur recovery is the Claus process.
The conventional Claus process includes a thermal combustion stage and a catalytic reaction stage. In terms of equipment, the thermal combustion stage includes a thermal reactor and heat recovery units. The catalytic reaction stage includes two or three catalytic reactors along with associated re-heaters and condensers.
In the thermal combustion stage hydrogen sulfide, sulfur dioxide, and oxygen react to convert hydrogen sulfide to elemental sulfur in the thermal reactor. Approximately 60-70% of the hydrogen sulfide entering the thermal combustion stage can be converted to elemental sulfur. The elemental sulfur is separated through condensation and the remaining gases are introduced to the catalytic section. In the catalytic section, additional sulfur is produced by further reactions in the presence of a catalyst at lower temperatures than in the thermal reactor.
The conventional Claus process typically recovers 95 percent (%) to 98% of the hydrogen sulfide depending on the number of catalytic converters. Sulfur recovery efficiencies for Claus plants with two Claus converters are between 90% and 97%. Sulfur recovery efficiencies are between 95% and 98% for a Claus process with three catalytic converters.
But, there is increasing demand to achieve higher sulfur removal and recovery efficiency due to tight emissions regulations. Recent environmental regulations regarding sulfur oxides (SOx) emissions require a sulfur recovery efficiency in the range of 98.5% to 99.9% or higher. Conventional Claus processes fail to meet this requirement due to the equilibrium nature of the reactions in the Claus process. The equilibrium nature of the reactions impedes the conversion in the conventional Claus process. In order to achieve increased sulfur recovery in a conventional Claus process, addition of a tail-gas treatment units (TGTU) is required.
The addition of a tail-gas treatment unit (TGTU) can increase sulfur recovery to or greater than 99.9%, but requires complex and expensive equipment. The TGTU entails either an add-on unit at the end of the Claus unit or a modification to the Claus unit itself. The add-on TGTU at the end of the Claus unit is generally used when the Claus process includes two Claus converters. Although there are several varieties of tail gas treatment technologies, they can be grouped into the following four broad categories: sub-dew point Claus process, direct oxidation of H2S to sulfur, sulfur dioxide (SO2) reduction and recovery of H2S, and H2S combustion to SO2 and recovery of SO2.
Sub-dew point Claus processes are processes based on a Claus converter performing at temperatures below the sulfur dew point (lower temperature is desirable due to equilibrium nature of the Claus catalytic reaction). Sub-dew point processes provide high equilibrium conversions in one catalyst bed, but are complicated by the need for periodic catalyst regeneration by sulfur evaporation at elevated temperatures. To accommodate for regeneration, such processes are usually performed in two or three (or even more) parallel reactors, periodically undergoing reaction and regeneration. Cold-bed-adsorption (CBA) is the most efficient sub-dew point process and can achieve 99% sulfur recovery.
Processes involving direct oxidation of H2S to sulfur are based on selective oxidation of H2S by oxygen to elemental sulfur using selective catalysts. An example of a direct oxidation method includes the SuperClaus process.
TGTU technology based on SO2 reduction and recovery of H2S involves the catalytic hydrogenation of leftover sulfur species to H2S, absorption of the H2S with amine solution and then recycling the H2S back to the Claus furnace. An example of an SO2 reduction and recovery of hydrogen sulfide process is the SCOT process.
TGTU technology based on H2S combustion to SO2 and recovery of SO2 involves the combustion of leftover H2S in the tail gas stream to SO2, absorption of SO2 with a solvent (wet scrubbing), and recycling the SO2 back to the feed to the Claus plant. Although SO2 scrubbing, also known as flue gas scrubbing, has not been commercially tested as a TGTU, the technology has been extensively used as flue gas scrubbing for coal based power stations.
Considering the complexity of a Claus process and the need for high cost TGTU to meet the environmental regulations, simpler and cheaper alternatives, which can be easily implemented and retrofitted to the existing units, is necessary and will be a significant contribution to the industry.